Progress toward all-digital medical imaging environments has substantially increased the speed at which large amounts of medical image information can be accessed and displayed to a radiologist. X-ray based imaging for breast cancer screening/diagnosis is a particularly important field that is experiencing such information-expanding technological progress. Historically, breast cancer screening/diagnosis has been based on conventional x-ray projection mammography techniques in which an x-ray source projects x-rays through a breast that is immobilized by compression against a breast platform. A two-dimensional projection image of the breast, referred to as a mammogram, is captured by a film or digital x-ray detector located beneath the breast platform.
Although conventional x-ray mammography is currently recognized as one of the best FDA-approved methods for detecting early forms of breast cancer, it is still possible for cancers to be missed during radiological viewing of the mammogram. A variety of factors, such as breast density, may contribute to the failure to detect breast cancers.
For these and other reasons, substantial attention and technological development has been dedicated toward breast x-ray tomosynthesis, which is similar in many respects to conventional x-ray mammography except that the x-ray source is no longer stationary, but instead rotates through a limited angle relative to the breast platform normal (e.g., −15 degrees to +15 degrees) while several projection images (e.g., 10-15 projection images) are acquired by the x-ray detector. The several projection images are then mathematically processed to yield a number (e.g., 40-60) of tomosynthesis reconstructed images, each corresponding to a different slice of breast tissue, which can then be examined by the radiologist. Whereas a particular cancerous lesion positioned within a region of dense fibroglandular tissue might have been obscured in a single conventional x-ray mammogram view, that lesion could be readily apparent within a set of tomosynthesis reconstructed images representative of individual slices through the dense fibroglandular tissue.
Although x-ray tomosynthesis imaging and computed tomography (CT) imaging are both generally considered to be three dimensional imaging modalities, there are differences between these two modalities that can have an impact on the way their associated data volumes are best processed and/or reviewed for the detection of anatomical abnormalities. Concordant with the favorability of x-ray tomosynthesis imaging over CT imaging from the perspectives of radiation dose and cost/complexity of the image acquisition equipment, the number and angular range of projection images for x-ray tomosynthesis imaging is less than for CT imaging, which requires a minimum angular span of at least 180 degrees plus a fan beam angle. However, unlike with CT imaging, x-ray tomosynthesis imaging is not capable of providing a “true” value for the x-ray absorption property of any particular point in the imaged volume, but instead only provides such value in inseparable combination with varyingly “blurred” versions of the absorption property from other parts of the imaged volume. The number of distinct reconstructed image slices containing useful and anatomically differentiating information is substantially less for x-ray tomosynthesis than for CT imaging, and x-ray tomosynthesis reconstructed images are often artifact-laden and highly anisotropic according to the particular range and orientation of the tomosynthesis imaging arc traversed. For these reasons, x-ray tomosynthesis imaging is sometimes termed a “quasi” three-dimensional imaging modality, with CT imaging representing a “true” three-dimensional imaging modality. While the preferred embodiments infra are particularly advantageous when applied to the peculiarities of x-ray tomosynthesis image volumes, it is nevertheless to be appreciated that one or more aspects of the described embodiments can be extended to the processing and display of CT data volumes without departing from the scope of the present teachings.
Computer-aided detection (CAD) refers to the use of computers to analyze medical images to detect anatomical abnormalities therein, and/or the use of computers to otherwise process image information in a manner that facilitates perception of the medical image information by a radiologist. Sometimes used interchangeably with the term computer-aided detection are the terms computer-aided diagnosis, computer-assisted diagnosis, or computer-assisted detection. CAD findings are most often communicated in the form of annotation maps comprising graphical annotations (CAD markers) overlaid on a diagnostic-quality or reduced-resolution version of the medical image. Substantial effort and attention has been directed to increasing the analysis capabilities of CAD systems, resulting in ever-increasing amounts of information that is available to the radiologist for review. Thousands of CAD systems for conventional x-ray mammography are now installed worldwide, and are used to assist radiologists in the interpretation of millions of mammograms per year.
Development and commercialization of CAD systems capable of identifying anatomical abnormalities in x-ray tomosynthesis data volumes also continues. However, in progressing from conventional x-ray mammography to breast x-ray tomosynthesis imaging, practical issues arise with regard to the rising volume of data requiring review by the radiologist. Whereas there are usually just four conventional x-ray mammogram images per patient, there can be hundreds of tomosynthesis reconstructed image slices (e.g., 40-60 slices for each of the four views). As more visual information becomes available, an important challenge is to present such information to the radiologist effectively and efficiently such that screening for abnormalities can be done thoroughly and effectively, and yet in a reasonable time to be practical, and diagnostic assessment can also be facilitated.
Of particular importance is the manner in which an image review workstation displays CAD markers to the radiologist in the large stack of tomosynthesis reconstructed images. For CAD markers displayed during a first reading, it is desirable that the CAD markers not be overly obtrusive on their corresponding image, it is also desirable that they not be readily overlooked as the radiologist moves through his/her examination of the image slices. For CAD markers displayed as part of a second reading, the CAD marks may be more obviously displayed, but due to the sheer volume of available tomosynthesis image slices, it is still possible that CAD markers may be overlooked. One problem that may be encountered when reviewing CAD markers in a tomosynthesis data set is that the markers are not located on all of the image slices. In fact, in a given set it may be that CAD markers are only located on a few of the images. One method of facilitating a more efficient CAD review during a radiological reading is described in the commonly assigned U.S. Pat. No. 7,630,533B2, which is incorporated by reference herein, and which describes a ruler identifying the slices for display. Each slice that contains a marker has an indicator positioned next to the ruler. With such an arrangement a reviewer can quickly identify a slice with a CAD mark and transition rapidly to the slice of interest by selecting the marker that is near the ruler, thereby increasing reviewing efficiency.
Another method of increasing the efficiency of CAD review during radiological reading is described in the commonly assigned US 2009/0087067A1, which is incorporated by reference herein, and which describes including CAD proximity markers on one or more image slices neighboring those that contain actual CAD markers, such that a reviewer who is quickly paging through many slices will be less likely to miss the CAD-marked slices. Both of the above techniques reduce the chance that an image slice will be overlooked during review, yet each still require sifting through multiple images to identify those images with the most relevant information. Other issues arise as would be readily apparent to one skilled in the art in view of the present disclosure.